GCN, nvptx: Fatal error for missing symbols in 'libhsa-runtime64.so.1', 'libcuda.so.1'

If 'libhsa-runtime64.so.1', 'libcuda.so.1' are not available, the corresponding
libgomp plugin/device gets disabled, as before.  But if they are available,
report any inconsistencies such as missing symbols, similar to how we fail in
presence of other issues during device initialization.

	libgomp/
	* plugin/plugin-gcn.c (init_hsa_runtime_functions): Fatal error
	for missing symbols.
	* plugin/plugin-nvptx.c (init_cuda_lib): Likewise.